If you were to compress all the advice written on weed control to a single maxim, it would be: Attack early in the game.
Getting weeds out of the garden at the start of the season, when they're most vulnerable, is a smart strategy for two reasons: It keeps annual weeds from forming seed heads and keeps perennial weeds from developing deeper roots.
The most important thing you can do is to prevent more seeds from developing. Here's why: Most weeds in your garden are annuals, and annual weeds are phenomenally prodigious seed producers. A single crabgrass plant, for example, can produce 100,000 seeds, according to Barbara Pleasant, author of The Gardener's Weed Book: Earth-Safe Controls. If you don't get rid of these intruders before they develop viable seeds, the number of foes you'll have to battle will increase every year, and you'll always be playing catch-up.
With perennial weeds, seeds aren't the only problem, because they produce fewer of them.
Instead, perennials ensure their own survival by developing extensive underground root systems and/or sending out runners aboveground. If you catch them young, perennial weeds can usually be pulled out of the ground easily. But once established, they can be next to impossible to get rid of, as anyone who has battled Bermuda grass or yellow nutsedge in flower beds will attest.
To keep annuals from setting seed, weed early and often. Slicing off the top of the plant is all you have to do; there's no need to remove the roots. Many gardeners find that the most efficient tool for this task is some type of scuffle hoe, which cuts weeds at or just below the soil surface.
You can use the same off-with-their-heads approach to reduce the amount of annual weeds in a lawn. Simply mow high (3 inches or so) and often (weekly or biweekly). This keeps new seeds from forming, and the taller grass shades the ground, preventing the germination of weed seeds already in the soil.
What about all the seeds already lurking in the soil, waiting to sprout?
The easiest way to control them is by never letting them see the light of day. If sunlight doesn't reach them, they don't germinate. Plant closely so that most of the soil is shaded by foliage, and mulch thickly so sunlight won't reach the soil. Avoid excessive cultivation, which brings weed seeds closer to the surface. In vegetable beds, don't leave the earth bare between crops: Put down a temporary mulch or plant a cover crop like clover that you can dig under later.
If you're preparing a new flower or vegetable bed, try another approach: forcing out the enemy. Till and amend the soil as you normally would prior to planting. Then keep the prepared soil moist for a few days to encourage weed seeds to sprout. When they do, hoe them off and water again. Repeat this process until there are few weed seeds left to sprout, leaving little weeding thereafter.
With perennials, it is essential to get all of the plant out of the ground. When perennial weeds are tender seedlings, they can usually be pulled up by hand with ease, especially when the soil is moist. But the more time you allow these weeds to take hold, the harder they are to eradicate. The least bit of Bermuda grass rhizome left behind can sprout into a new plant.